单项选择题 A rotary current sets through all directions of the compass. The time it takes to complete one of these cycles is approximately（）.

单项选择题 A light，feathery deposit of ice caused by the sublimation of water vapor directly into the crystalline form，on objects whose temperatures are below freezing，is called（）.

单项选择题 A current will develop between areas of different density in ocean waters. If you face in the same direction the current is flowing，the water of（）.